diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 57a464345c7144f927765f00f1ce9601f8ec30cc..c39304c465b5889c3ff93a4a9d13f7f4786efe35 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,9 @@
+2009-12-28  Jason Merrill  <jason@redhat.com>
+
+	PR c++/42447
+	* pt.c (iterative_hash_template_arg): Don't rely on TYPE_CANONICAL
+	for ARRAY_TYPE.
+
 2009-12-24  Jason Merrill  <jason@redhat.com>
 
 	PR c++/41305, DR 384
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 85ad539f419251881cdf2c0fbb4cf0039cf013c6..d5342a1653fd0700b486a49bb45dfb6c4a8e9bd6 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -1553,6 +1553,13 @@ iterative_hash_template_arg (tree arg, hashval_t val)
       val = iterative_hash_object (code, val);
       return iterative_hash_template_arg (TREE_OPERAND (arg, 2), val);
 
+    case ARRAY_TYPE:
+      /* layout_type sets structural equality for arrays of
+	 incomplete type, so we can't rely on the canonical type
+	 for hashing.  */
+      val = iterative_hash_template_arg (TREE_TYPE (arg), val);
+      return iterative_hash_template_arg (TYPE_DOMAIN (arg), val);
+
     default:
       switch (tclass)
 	{
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index aef38ad70f5135fd473e775ea2956859087a4c2f..179b08cb36b44f8cc1c270f7e694836f72e97062 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2009-12-28  Jason Merrill  <jason@redhat.com>
+
+	PR c++/42447
+	* g++.dg/template/array21.C: New.
+
 2009-12-28  Janus Weil  <janus@gcc.gnu.org>
 
 	PR fortran/42353
diff --git a/gcc/testsuite/g++.dg/template/array21.C b/gcc/testsuite/g++.dg/template/array21.C
new file mode 100644
index 0000000000000000000000000000000000000000..5c5f2f65d6b7be412c0f7ec75b6926a4e22eb8ca
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/array21.C
@@ -0,0 +1,50 @@
+// PR c++/42447
+
+template<int>
+  void* get(int);
+
+template<typename>
+  struct unique_ptr;
+
+template<typename _Tp>
+  struct unique_ptr<_Tp[]>
+  {
+    typedef int __tuple_type;
+
+    void*
+    get() const
+    { return ::get<0>(_M_t); }
+
+    __tuple_type _M_t;
+  };
+
+template <typename T> class dynamic_dispatch;
+
+template <typename TC>
+  struct dynamic_dispatch<void (TC::*)(int&)>
+  {
+    struct entry { };
+    unique_ptr<entry[]> m_Start;
+
+    template <typename UC>
+      void attach_handler(void (UC::*m)(int&))
+      {
+        entry* p = 0;
+        do {
+        } while(--p != m_Start.get());
+      }
+  };
+
+template <typename TC>
+  class request_dispatcher
+  : private dynamic_dispatch<void (TC::*)(int&)>
+  { request_dispatcher(); };
+
+struct file_reader
+{
+  void execute_command(int&);
+};
+
+template <>
+  request_dispatcher<file_reader>::request_dispatcher()
+  { this->attach_handler(&file_reader::execute_command); }